The invention relates to a boron suboxide composite and to a method for its preparation.
The first laboratory synthesis of diamond triggered extensive efforts to design and develop materials with a combination of properties approaching or even improving upon those of diamond. The best known of these superhard materials is cubic boron nitride (cBN). It is also known that boron rich compounds provide good candidates for this type of application. They give rise to a large family of refractory materials with unique crystal structures and a range of interesting physical and chemical properties related to their short interatomic bond lengths and their strongly covalent character. Boron rich phases with a structure based on that of α-rhombohedral boron include boron carbide and boron suboxide (nominally B6O), which combine high hardness with low density and chemical inertness, making them useful as abrasives and for other high-wear applications [1 ].
The boron suboxide (B6O) structure, space group R 3m, consists of eight B12 icosahedral units situated at the vertices of a rhombohedral unit cell. The structure can be viewed as a distorted cubic close packing (ccp) of B12 icosahedra. Two O atoms are located in the interstices along the [111] rhombohedral direction.
The synthesis of boron suboxide (B6O) and a description of its properties have been extensively reported in the literature, even though pure material with a high degree of crystallinity is difficult to synthesize. Boron suboxide materials formed at or near ambient pressure are generally oxygen deficient (B6Ox, x<0.9). They also have poor crystallinity and very small grain size. High pressure applied during the synthesis of B6O can significantly increase the crystallinity, oxygen stoichiometry, and crystal size of the products [1]. Although boron suboxide is reported as the nominal composition B6O, it is widely accepted to be non-stoichiometric. For brevity, the nominal formula B6O is used in this specification.
In U.S. Pat. No. 3,660,031 a method of preparing boron suboxide is disclosed. According to this disclosure, the boron suboxide is formed by reducing zinc oxide with elemental boron at a temperature in the range of 1200° C. to 1500° C. It is reported as having the formula B7O, and is also characterized as having an average hardness value of 38.20 GPa under a load of 100 g, and a density of 2.60 g/cm3. The fracture toughness of this material is not reported.
U.S. Pat. No. 3,816,586 also discloses a method of fabricating boron suboxide. According to this disclosure, boron suboxide is formed by hot pressing the mixture of elemental boron and boron oxide at suitable temperatures and pressures. Upon analysis, the boron suboxide product is said to have given 80.1 wt. % boron and 19.9 wt. % oxygen which corresponds to the stoichiometry of B6O. It is also reported as having a density of 2.60 g/cm3 and a Knoop hardness under a 100 g load (KNH100) of 30 GPa. The fracture toughness of this material is not reported.
A great deal of research has shown that while boron suboxide material has a very high hardness its fracture toughness is very low, i.e. the material is brittle. From the literature, Itoh et. al. [2], B6O compacts have been manufactured at high temperatures (1400° C.-1800° C.) and high pressures (3-6 GPa). This B6O powder is reported to have been synthesized from elemental boron and boric oxide. Upon analysis, the B6O compacts are reported as having an average hardness of 31-33 GPa and a very low fracture toughness. Itoh et. al. [3, 4] and Sasai et. al. [5], have also tried to improve the mechanical properties of B6O, especially fracture toughness, using other hard materials like cBN [3], boron carbide [4], and diamond [5], respectively. The hardness for these B6O composites is respectable but the fracture toughness is reported to be still low, B6O-diamond composites having a fracture toughness of about 1 MPa·m0.5, B6O-cBN composites having a fracture toughness of about 1.8 MPa·m0.5 and B6O—B4C composites having a fracture toughness of about 1 MPa·m0.5.
It is an object of the present invention to provide a method of producing B6O composites with a respectable hardness as well as a better fracture toughness, compared to the previously reported B6O composites.